Micro-optical array projectors are discussed as replacement for structured illumination in applications with critical space requirements. The concept bases on the fly’s eye condenser principle and a well-defined buried array of micro-dia. Their optical performance benefits from etendue conservation and the large depth of field of applied short focal lenses. As an established technique to generate microlens profiles, thermal reflow of binary patterned photoresist is known for more than three decades. This approach leads to lens arrays with filling factors up to ~90% when used in hexagonal arrangement. Further increment requires direct writing methods such as grayscale lithography. Recently a LED based projection stepper-like lithography system became competitive, because it allows structure depths beyond 50 microns. It utilizes an LCoS micro-imager as variable 8-bit reticle and a high dynamic dosage controlled illumination. This paper represents the evaluation of the technique for the generation of refractive lens profiles by means of metrology and optical performance of micro-optical array projectors. Micro-array projectors based on circular lenslets will be compared, followed by the analysis of closely packed square-shaped lenslets. The aim is to understand the impact of lens shape deviation, conical constant or statistical distribution of lens properties like sag height, radius of curvature on the projection. A correlation of imperfections and quality loss due to scattering, aberrations, and mismatch of images in the overlay of different projectorlets will be given. The work concludes with an outlook on further developments in mastering micro-optical profiles for illumination application.
We developed a novel LED projection based direct write grayscale lithography system for the generation of optical surface profiles such as micro-lenses, diffractive elements, diffusors, and micro freeforms. The image formation is realized by a LCoS micro-display which is illuminated by a 405 nm UV High Power LED. The image on the display can be demagnified from factors 5x to 100x with an exchangeable lens. By controlling exposure time and LED power, the presented technique enables a highly dynamic dosage control for the exposure of h-line sensitive photo resist. In addition, the LCoS micro-display allows for an intensity control within the micro-image which is particularly advantageous to eliminate surface profile errors from stitching and limited homogeneity from LED illumination. Together with an accurate calibration of the resist response this leads to a superior low surface error of realized profiles below <0.2% RMS. The micro-display is mounted on a 3-axis (XYθ) stage for precise alignment. The substrate is brought into position with an air bearing stage which addresses an area of 500 × 500 mm<sup>2</sup> with a positioning accuracy of <100 nm. As the exposure setup performs controlled motion in the z-direction the system to maintain the focal distance and lithographic patterning on non-planar surfaces to some extent. The exposure concept allows a high structure depth of more than 100 μm and a spatial resolution below 1 μm as well as the possibility of very steep sidewalls with angles larger than >80°. Another benefit of the approach is a patterning speed up to 100 cm<sup>2</sup>/h, which allows fabricating large-scale optics and microstructures in an acceptable time. We present the setup and show examples of micro-structures to demonstrate the performance of the system, namely a refractive freeform array, where the RMS surface deviation does not exceed 0.2% of the total structure depth of 75 μm. Furthermore, we show that this exposure tool is suitable to generate diffractive optical elements as well as freeform optics and arrays with a high aspect ratio and structure depth showing a superior optical performance. Lastly we demonstrate a multi-level diffraction grating on a curved substrate.
The combination of a 10.6 μm main pulse CO<sub>2 </sub>laser and a 1064 nm pre-pulse Nd:YAG laser in EUV source concepts for HVM would require collector mirrors with an integrated spectral purity filter that suppresses both laser wavelengths. This paper discusses a new approach of a dual-wavelength spectral purity filter to suppress 10.6 μm and 1064 nm IR radiation at the same time. The dual-wavelength spectral purity filter combines two binary phase gratings that are optimized for 10.6 μm and 1064 nm, respectively. The dual phase grating structure has been realized on spherical sub-aperture EUV collector mirrors having an outer diameter of 150 mm. IR suppression factors of 260 at 10.6 μm and 620 at 1064 nm have been measured on the sub-aperture EUV collector while its EUV reflectance exceeded 64 % at 13.5 nm.
Modern applications in biomedical imaging, machine vision and security engineering require close-up optical systems with high resolution. Combined with the need for miniaturization and fast image acquisition of extended object fields, the design and fabrication of respective devices is extremely challenging. Standard commercial imaging solutions rely on bulky setups or depend on scanning techniques in order to meet the stringent requirements. Recently, our group has proposed a novel, multi-aperture approach based on parallel image transfer in order to overcome these constraints. It exploits state of the art microoptical manufacturing techniques on wafer level in order to create a compact, cost-effective system with a large field of view. However, initial prototypes have so far been subject to various limitations regarding their manufacturing, reliability and applicability. In this work, we demonstrate the optical design and fabrication of an advanced system, which overcomes these restrictions. In particular, a revised optical design facilitates a more efficient and economical fabrication process and inherently improves system reliability. An additional customized front side illumination module provides homogeneous white light illumination over the entire field of view while maintaining a high degree of compactness. Moreover, the complete imaging assembly is mounted on a positioning system. In combination with an extended working range, this allows for adjustment of the system’s focus location. The final optical design is capable of capturing an object field of 36x24 mm<sup>2</sup> with a resolution of 150 cycles/mm. Finally, we present experimental results of the respective prototype that demonstrate its enhanced capabilities.
Natural compound-eyes consist of a large number of ommatidia that are arranged on curved surfaces and thus are able to detect signals from a wide field of view. We present an integrated artificial compound-eye sensor system with enhanced field of view of 180° × 60° due to the introduction of curvature. The system bases on an array of adaptive logarithmic wide-dynamic-range photoreceptors for optical flow detection and compound-eye optics for increasing sensitivity and expanding the field of view. Its assembling is mainly done in planar geometry on a flexible printed circuit board. The separation into smaller ommatidia blocks by dicing enables flexibility and finally allows for mounting on curved surfaces. The signal processing electronics of the presented system is placed together with further sensors into the concavity of the photoreceptor array, and facilitates optical flow computation for navigation purposes.
Wafer-level optics is considered to yield imaging lenses for cameras of the smallest possible form factor. The high accuracy of the applied microsystem technologies and the parallel fabrication of thousands of modules on the wafer level make it a hot topic for high-volume applications with respect to quality and costs. However, the adaption of existing materials and technologies from microoptics for the manufacturing of millimeter scale lens diameters led to yield problems due to material shrinkage and z-height accuracy. A multi-aperture approach to real-time vision systems is proposed that overcomes these issues because it relies on microlens arrays. The demonstrated prototype achieves VGA (Video Graphics Array, 640×480 pixels) resolution with a thickness of 1.4 mm, which is a thickness reduction of 50% compared to single-aperture equivalents. The partial images that are separately recorded in different channels are stitched together to form a final image of the whole field of view by means of image processing. Distortion is corrected within the processing chain. The microlens arrays are realized by state-of-the-art micro-optical fabrication techniques on wafer level that are suitable for a potential application in high volume, e.g., for consumer electronic products.
Micro-optical systems, that utilize multiple channels for imaging instead of a single one, are frequently discussed for
ultra-compact applications such as digital cameras. The strategy of their fabrication differs due to different concepts of
image formation. Illustrated by recently implemented systems for multi-aperture imaging, typical steps of wafer-level
fabrication are discussed in detail. In turn, the made progress may allow for additional degrees of freedom in optical
design. Pressing ahead with very short overall lengths and multiple diaphragm array layers, results in the use of
extremely thin glass substrates down to 100 microns in thickness. The desire for a wide field of view for imaging has led
to chirped arrays of microlenses and diaphragms. Focusing on imaging quality, aberrations were corrected by
introducing toroidal lenslets and elliptical apertures. Such lenslets had been generated by thermal reflow of lithographic
patterned photoresist and subsequent molding. Where useful, the system's performance can be further increased by
applying aspheric microlenses from reactive ion etching (RIE) transfer or by achromatic doublets from superimposing
two moldings with different polymers. Multiple diaphragm arrays prevent channel crosstalk. But using simple metal
layers may lead to multiple reflections and an increased appearance of ghost images. A way out are low reflecting black
matrix polymers that can be directly patterned by lithography. But in case of environmental stability and high resolution,
organic coatings should be replaced by patterned metal coatings that exhibit matched antireflective layers like the
prominent black chromium. The mentioned components give an insight into the fabrication process of multi-aperture
imaging systems. Finally, the competence in each step decides on the overall image quality.
The miniaturization of actuators results in two major consequences: First, the reduction in efficiency depending on the
physical principle. Second, the increasing requirements in positioning accuracy during the assembly and fabrication
process in combination with low cost production.
Electrostatic polymeric actuators providing out-of-plane motion which can be completely fabricated in parallel
fabrication steps and hence be produced on wafer level are compliant to these tolerance and cost constraints.
The electrostatic actuation principle is a surface effect and therefore independent of the volume. In addition, the
efficiency of electrostatic actuation increases with a decreasing gap size between the electrodes. The simple morphology
of such actuators can be easily produced by UV-replication of polymeric materials. In consequence, the electrostatic
actuation principle is predestined for the combination with low cost wafer level fabrication.
This paper reports on the first results of successfully fabricated electrostatic actuators produced on wafer level. Instead of
using a standard silicon substrate our approach is based on the lithographic structuring of the non-conducting material
ORMOCER®. In comparison ORMOCER® has a significantly lower elastic modulus (about 1 GPa). Therefore, only a
fraction of actuation voltage is necessary for a similar deflection. The material is structured using photolithography and
the electrodes are realized with coatings of thin metal layers. Experimental results show a deflection up to 49,1 μm at
500 V for an 75 μm thick cantilever beam fixed at both ends. Good agreement between measurements and simulations is
achieved, proving the applicability of the software and the assumed material parameters.
We propose a microoptical approach to ultra-compact optics for real-time vision systems that are inspired by the
compound eyes of insects. The demonstrated module achieves about VGA resolution with a total track length
of 1.4 mm which is about two times shorter than comparable single aperture optics. The partial images that
are separately recorded in different optical channels are stitched together to form a final image of the whole field
of view by means of image processing. A software correction is applied to each partial image so that the final
image is made free of distortion. The microlens arrays are realized by state of the art microoptical fabrication
techniques on wafer-level which are suitable for a potential application in high volume e.g. for consumer electronic
As a matter of course, cameras are integrated in the field of information and communication technology. It can
be observed, that there is a trend that those cameras get smaller and at the same time cheaper. Because single
aperture have a limit of miniaturization, while simultaneously keeping the same space-bandwidth-product and
transmitting a wide field of view, there is a need of new ideas like the multi aperture optical systems. In the
proposed camera system the image is formed with many different channels each consisting of four microlenses
which are arranged one after another in different microlens arrays. A partial image which fits together with the
neighbouring one is formed in every single channel, so that a real erect image is generated and a conventional
image sensor can be used. The microoptical fabrication process and the assembly are well established and can
be carried out on wafer-level. Laser writing is used for the fabrication of the masks. UV-lithography, a reflow
process and UV-molding is needed for the fabrication of the apertures and the lenses. The developed system is
very small in terms of both length and lateral dimensions and has a VGA resolution and a diagonal field of view
of 65 degrees. This microoptical vision system is appropriate for being implemented in electronic devices such
as webcams integrated in notebookdisplays.
Although several applications of machine vision and biomedical imaging ask for the close-up imaging of extended
object fields, only few, mostly bulky solutions exist. We demonstrate the optical design and realization of an
ultra-compact close-up imaging system with unity magnification. It uses a multi-aperture approach in order to
shorten its total track length to less than 4 mm while achieving a large field of view. The system is made of a stack
of several two-dimensional arrays of refractive microlenses. The potential of this setup lies in the combination of
digital imaging with microoptical fabrication techniques leading to thin optical components which can be directly
attached to an image sensor. Hence, these systems fit into tight spaces and they achieve a high resolution without
The integration of camera modules in portable devices is increasing rapidly. At the same time, their size is shrinking due
to the need for mobility and reduction of costs. For this purpose, an ultra-compact imaging system has been realized,
which adapts the multichannel imaging principle of superposition compound eyes known from nocturnal insects. The
application forms an erect image by using a pair of microlens arrays with slightly different pitches, which is also known
as "Gabor superlens". The microoptical design was optimized by using numerical ray tracing methods with respect to the
capabilities of state-of-the-art microoptics fabrication technology. Additional aperture/diaphragm layers and a field lens
array had to be introduced in order to avoid channel cross talk. As a result, the optical performance is comparable to that
of miniaturized conventional lens modules. However, the fabrication of the microoptical Gabor superlens is kept simple
and scalable in terms of wafer-level technology due to the use of microlens arrays with low sag heights and small
Up to now, multi channel imaging systems have been increasingly studied and approached from various directions
in the academic domain due to their promising large field of view at small system thickness. However, specific
drawbacks of each of the solutions prevented the diffusion into corresponding markets so far. Most severe problems
are a low image resolution and a low sensitivity compared to a conventional single aperture lens besides the lack
of a cost-efficient method of fabrication and assembly. We propose a microoptical approach to ultra-compact
optics for real-time vision systems that are inspired by the compound eyes of insects. The demonstrated modules
achieve a VGA resolution with 700x550 pixels within an optical package of 6.8mm x 5.2mm and a total track
length of 1.4mm. The partial images that are separately recorded within different optical channels are stitched
together to form a final image of the whole field of view by means of image processing. These software tools allow
to correct the distortion of the individual partial images so that the final image is also free of distortion. The
so-called electronic cluster eyes are realized by state-of-the-art microoptical fabrication techniques and offer a
resolution and sensitivity potential that makes them suitable for consumer, machine vision and medical imaging
A new technology based on plasma etching has been developed to produce antireflective surface structures. By choosing thin initial layers and variable plasma conditions, a broad range of nanostructures can be obtained on various polymers. A broadband antireflective effect can be achieved that is less sensitive to the incident angle of light compared to multilayer interference coatings. Thin layers of silica help in mechanical protection, especially if the structured surface is nearly enclosed by the protection layer. In addition, surfaces that show both antireflective properties and an antifogging effect have been prepared. Combinations of SiO<sub>2</sub> and fluorine-containing layers were found to be useful in obtaining
super-hydrophobic behavior. This advanced plasma etching is not limited to a special plasma source and the suitability of different plasma sources is discussed.
High transparent thermoplastics have the capability to put glass out of business, especially in everyday life's optics. Their diverse nature gives rise to different antireflection principles. The reduction of surface reflection losses in polymethylmethacrylate (PMMA) is demonstrated by means of argon/oxygen plasma treatment. Since the presented reduction of reflection occurs in a wide spectral range, the technique may be applied for omnidirectional devices or curved substrates. The etching process creates a self-organized stochastic subwavelength structure at the substrate itself. The decrease in reflection is described by effective medium theory (EMT), converting the surface topology into a depth-dependent filling factor profile. In a second step this nano-scaled structure is used as the initial point for a broadband absorber by coating it with a nontransparent metal layer. A high-efficient absorber can be obtained, if the metal acts as backside coating of the double-sided plasma-treated substrate and steady-going transitions between the materials eliminating the Fresnel reflections. In practice, the magnitude of absorption depends on depth of structure as well as on the complex refractive index of the metal.
Inhomogeneous coatings are promising for superior optical properties, e.g. broadband antireflection, in comparison to conventional HL-stack designs. Although a lot of excellent theoretical work on optical behaviour of rugates and gradient index films has been done during the last decades, there is no real breakthrough in industrial fabrication. The realization of such coatings leads to an extensive and time-consuming computer-aided control, because of complicated layer designs with continuously changing refractive index gradients. We describe the design and optical performance of an omnidirectional antireflection coating that essentially represents a hybrid coating composed from homogeneous layers and linear refractive index gradient layers.